rm is a remove or delete command. Linux, like other UNIX variants assumes you know what you are doing and will allow you to totally wreck the system as root.
rm -rf * will delete EVERYTHING in the filetree from where you currently are.
echo "stuff" > myfile will overwrite myfile instead of appending unlike
echo "stuff" >> myfile. This is a common mistake and very frustrating.
|The > and >> are IO redirection|
dd is a type of file copy command with (as root) the ability to write anywhere on a hard drive with no regard to files or file structure.
dd if=/dev/zero of=/dev/sda bs=512 count=1 will obliterate the master boot record of the hard drive sda making the system unbootable.